A comprehensive study of two-phase flow mechanisms and pressure drop in horizontal small hydraulic diameter tubes was conducted. Co-current flow of air-water mixtures in four round tubes and one rectangular tube with hydraulic diameters ranging from 5.5 mm to 1.3 mm were investigated. Bubble, dispersed, elongated bubble, slug, stratified, wavy, annular-wavy, and annular flow patterns were observed. The results of this work show that diameter and surface tension effects play an important role in determining the flow patterns and transitions between them;Flow mechanisms during condensation of refrigerant R134a in small diameter round, square and rectangular tubes (0.506 mm < Dh < 4.91 mm) were also investigated. Flow mechanisms were recorded and categorized into intermittent, wavy, annular, and dispersed flow over the entire range of qualities, and for five different refrigerant mass fluxes. As the hydraulic diameter is decreased, the influence of gravity diminishes and surface tension becomes more significant, thus promoting annular and slug/plug flow, and virtually eliminating the wavy flow regime. Transition lines between the flow patterns and regimes were established based on the experimental data. Many of the significant transition lines can be represented or approximated by constant Froude number lines, both for air-water mixtures, and refrigerant R134a. This common non-dimensional basis for transitions in fluids of widely different phase properties could be useful for extending the transition criteria to other fluids, geometries and operating conditions;Two-phase pressure drop measurements were taken on a set of 5 circular tubes and on 7 non-circular tubes (triangular, square, rectangular, barrel, and "N" shaped extruded tubes). Frictional components of the total measured two-phase pressure drops were determined by accounting for the small contributions due to expansion/contraction at the headers, and the deceleration component due to momentum change. Reasonable agreement was found between the pressure drops measured in this study for the larger tubes and correlations in the literature. Flow regime-based pressure drop correlations were developed for the following regimes: intermittent and discrete wave flows, annular and disperse wave flows, annular/mist flow, and mist flow. For each of these regimes, one correlation accounted for the circular geometries, while equations of the same functional form were developed for the non-circular geometries. It was found that the circular tube correlations were able to predict 90 percent of the data within 20 percent, while 92 percent of the non-circular tube data were predicted within 20 percent.